Yahoo email went through an update in 2013. By request from a web client, here is how to Update or add an email signature with a hot link.

1. Log into Yahoo Mail
2. Hover over the gear link in upper right and choose Mail Options:
   
3. In left menu choose Signature.  If you want a working link chose “Show a rich text signature“:
   
4. Compose your signature. (The default text seems huge so you might want to adjust the font size. )Highlight the text that you want to be a hot link and click the link icon:
   
5. Add your URL  and click OK
   
6. SAVE the signature
   
7. Compose a new email – your signature should be showing. If it is not, go back and SAVE the signature again (this happened to me). Two times was the charm.
8. To DELETE the signature, it is not good enough to remove the text and save, you have to chose the “Don’t use a signature” before you SAVE.